1. Field of the Invention
The present invention relates to a photoelectric conversion device and, more particularly, to a photoelectric conversion device used to read images in, e.g., a facsimile device, digital copying machine, digital camera, and the like.
2. Related Background Art
Conventionally, as an image reading system for a facsimile device, digital copying machine, or the like, or an image pickup apparatus for a video camera, digital camera, or the like, CCDs have been prevalently used. In recent years, a so-called amplification type photoelectric conversion device which has an amplification function of a MOS transistor or bipolar transistor in units of pixels has been developed extensively.
In order to realize high sensitivity in an amplification type photoelectric conversion device, noise removal is required. A photoelectric conversion device for noise removal described in Japanese Patent Laid-Open Application No. 9-205588 will be explained below. FIGS. 1A and 1B are respectively a circuit diagram and timing chart of the photoelectric conversion device. As shown in FIG. 1A, the photoelectric conversion device is characterized by having:                a photodiode 101 and MOS transistors 103 and 103′ which construct a photoelectric conversion unit, and a MOS switch 103 serving as a reset means for the photoelectric conversion unit;        MOS transistors 104, 106, and 106′, and a capacitor (CM) 105, which construct a noise signal holding unit for holding a noise signal upon resetting the photoelectric conversion unit; and        a noise signal removing unit (107 to 116) for removing noise signal using the held noise signal from the signal accumulated by the photoelectric conversion unit after that unit is reset.        
Also, a photoelectric conversion device, which has:                a MOS switch 107 and capacitor (CTN) 109 which construct a noise signal read-out unit for reading out a noise signal charge immediately after the reset;        a MOS switch 108 and capacitor (CTS) 110 which construct an optical signal read-out unit for reading out an optical signal charge after the optical signal is accumulated; and        a shift register 113 which serves as a scanning unit for sequentially scanning the noise signal from the noise signal read-out unit, and the optical signal from the optical signal read-out unit, and        which makes the scanning unit read out signals from the noise signal read-out unit (107, 109) and optical signal read-out unit (108, 110) and makes the photoelectric conversion unit accumulate an optical signal at the same time, is characterized by including:                    a noise signal holding unit (104, 105, 106, 106′) for holding a noise signal immediately after the reset until the optical signal accumulated after the reset is read out by the optical signal read-out unit (108, 110); and            buffer amplifiers 114 and 114′ and a differential amplifier 15 which construct a unit for outputting a difference between the held noise signal immediately after reset, and the optical signal after the reset.                        
Note that the MOS transistors 106 and 106′, and 103 and 103′ respectively form MOS source-follower circuits.
Inputs 116 and 116′ of the buffer amplifiers 114 and 114′ are common output lines, and circuits other than the buffer amplifiers 114 and 114′ and differential amplifier 115 are prepared in correspondence with the number of bits required.
In this photoelectric conversion device, all the circuits illustrated in FIG. 1A are formed on a single semiconductor substrate.
The operation and arrangement of the photoelectric conversion device will be explained below with reference to the timing chart in FIG. 1B.
Upon receiving a start pulse SP, the capacitors CTS 110 and CTN 109 for respectively accumulating an optical signal and noise signal are reset first.
Subsequently, a drive pulse φTN is turned on to read out a noise signal held by the capacitor CM 105 to the capacitor CTN 109. At this time, the noise signal read out from the capacitor CM 105 is a noise signal obtained immediately after a sensor was reset in the previous field. After the noise signal is read out to the capacitor CTN 109, a drive pulse φT1 is turned on to read out an optical signal to the capacitor CM 105. Furthermore, a drive pulse φTS is turned on to read out the optical signal to the capacitor CTS 110.
After that, a drive pulse φR is turned on to reset the sensor. Subsequently, the drive pulse φT1 is turned on to read out a signal immediately after the sensor was reset to the capacitor CM 105 as a noise signal. Then, the sensor starts accumulation.
Simultaneously with accumulation by the sensor, the optical signal and noise signal held by the capacitors CTS 110 and CTN 109 are sequentially output onto the common output line. Finally, a difference signal between the optical and noise signals is obtained by a differential circuit (not shown) or the like, and is output as a net optical signal.
Hence, in the present invention, a noise signal obtained at a sensor reset timing (1) shown in the timing chart is held in the capacitor CM 105 during the accumulation period (A), and is input to the capacitor CTN 109 before the optical signal is read out (A′). Hence, the difference between the noise signal (A′) and optical signal (B′) obtained at the identical sensor reset timing can be output as a net optical signal, sensor reset noise can be completely removed.
Also, the noise removing unit can be used not only in a photoelectric conversion device but also in a clamp circuit and the like.
However, in the above-mentioned photoelectric conversion device, it is often more important to read out photocharges at high speed by adjusting their accumulation start and end times than to attain smaller noise components. More specifically, for example, upon picking up an electronic flash light-control signal, an auto-focus (AF) signal, or the like, a high-speed read-out process is required rather than high S/N ratio of the signal.